An ever-present concern in the operation of electrical power generating stations is the production of sulfur dioxide gas as an end product of the combustion process. Sulfur dioxide production is of particular concern where it is necessary to use high sulfur fuel oil or coal as the fuel for the power generating facility.
Many processes have been proposed for removing sulfur dioxide from the flue gas prior to exhausting it to the atmosphere. Some such processes are effected by contacting the flue gas with an absorption liquid such as calcium hydroxide or calcium carbonate. Although efficient sulfur dioxide removal can be obtained by lime scrubbing of the flue gases, the end product of the scrubbing step is normally a calcium sulfite sludge which presents significant waste disposal problems, especially with large power generation facilities.
It has therefore been proposed to recover calcium sulfate dihydrate from the scrubbing process instead of the commercially valueless calcium sulfite sludge. Calcium sulfate dihydrate, known as gypsum may be used in its uncalcined form (CaSO.sub.4.2H.sub.2 O) as a portland cement retarder, as an agricultural soil additive, in the manufacture of plate glass, plasters and wallboard and in its calcined form (CaSO.sub.4.1/2H.sub.2 O) in plasters, Keenes cement wallboard manufacturer and as lath.
Calcium sulfate dihydrate may be obtained from calcium sulfite by oxidizing the calcium sulfite to calcium sulfate at a relatively low pH, e.g. 3.0-5.0. In Japanese Pat. No. 694017 issued June 19, 1973, the disclosure of which is incorporated herein by reference, it is noted that the solubility of the calcium sulfite generated in a lime absorption process such as described in Japanese Public Disclosure No. 47-42377 is much more soluble in a solution having a pH of about 4.0 than in solutions having pH's above 6.5. In Public Disclosure No. 47-42377 published Dec. 15, 1972 in the Japanese Patent Gazette, which is incorporated herein by reference, sulfur oxides are removed from flue gases by lime scrubbing in two successive scrubbing towers. The absorption liquid in a first most upstream absorption tower has a pH of 3.5 to 4.5 and the absorption liquid in a second absorption facility has a pH of 6.5 to 7.5. In Japanese Pat. No. 694017 it is suggested that the sulfite slurry of the first absorption tower of Disclosure No. 47-42377 be withdrawn and processed in an oxidation tower wherein the calcium sulfite is oxidized to calcium sulfate. The calcium sulfate may then be recovered as gypsum.
Calcium sulfate dihydrate (CaSO.sub.4.2H.sub.2 O) is a member of the monoclinic crystalline system and normally forms long needle-like acicular crystals when precipitated from solutions. Applicant has determined that such crystalline calcium sulate dihydrate formed by the oxidation of calcium sulfite obtained from the lime slurry scrubbing of flue gases containing sulfur oxides is unsuitable as a substitute for quarried, naturally occurring gypsum used in the manufacture of wallboard. That use accounts for 30 percent of the total United States gypsum use.
In the manufacture of wallboard, gypsum of high purity (at least about 95% CaSO.sub.4.2H.sub.2 O by weight) and low free water content (less than 1% by weight) is first obtained from naturally occurring deposits by quarrying. The quarried gypsum is then crushed, ground and milled as necessary prior to being fed to a calcining step where the dihydrate is converted to the hemi-hydrate (CaSO.sub.4.1/2H.sub.2 O). The calcined gypsum is then rewatered to form a slurry having a viscosity suitable for feeding to the wallboard machines.
In the wallboard machine the gypsum slurry is deposited in a continuous paper mold formed from a continuous sheet of adhesive coated paper which is scored and folded by the machine. A cover sheet is applied and after a residence time of about 5 minutes the gypsum has set and bonded to the paper sufficiently to allow it to be cut into desired lengths. The cut sheets are then dried to obtain the finished product.
In order for gypsum which has been formed by crystallization from solution to be acceptable, from both a physical and economic standpoint, as a substitute for quarried gypsum in the manufacture of wallboard several criteria must be met. First, in order to satisfy material handling and calcining requirements the gypsum is preferably supplied to the battery limits of the wallboard plant with less than 5 percent free moisture, although higher levels of up to about 25% can be processed where justified by economics. Secondly, the gypsum must be at least about 95% percent calcium sulfate dihydrate. Thirdly, the level of impurities such as soluble salt, total phosphate and total sulfite must be sufficiently low so that the paper bonding characteristics of the gypsum are not adversely affected. Fourthly, the normal consistency (the amount of water which must be added to a given weight of the calcined gypsum to provide a paste of the spcified viscosity) must be within acceptable economic limits, preferably less than 80 cc's of water per 100 grams of calcined gypsum. Lastly, in order to provide for economic transportation and storing, the bulk density should be at least 50 pounds per cubic foot at less than 1 percent free water content.
Of the foregoing criteria, one of the more critical is the normal consistency/viscosity requirement. Since any water added to the calcined gypsum must be removed in the wallboard drying step, when comparatively large amounts of water are required in order to produce a paste of satisfactory viscosity the substitute gypsum becomes economically unacceptable. This results both from increased drying costs and increased water usage. In a period of shortages of both energy and water, failure to meet normal consistency requirements would eliminate a manufactured gypsum from consideration as a substitute for naturally occurring gypsum.
It is therefore an object of the present invention to crystallize from solution a calcium sulfate dihydrate which is suitable for use in the wallboard manufacturing process.
It is a further object of the invention to produce a calcium sulfate dihydrate of high purity.
It is yet a further object of the invention to provide a calcium sulfate dihydrate product having a modified, generally rhombic crystal structure which exhibits normal consistency characteristics similar to or better than those exhibited by naturally occurring gypsum.
It is a still further object of the invention to provide a calcium sulfate dihydrate product having a bulk density comparable to that of naturally occuring gypsum.
These and other objects of the invention are obtained by the modified calcium sulfate dihydrate of the invention and by the process of the invention for making the modified crystal.